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Abstract:

A specific type of carbazole photoinitiator is capable of providing
radiation curable compositions that are curable by UV LEDs and do not
exhibit an unstable yellowing behaviour in an image upon storage like
ITX.

Claims:

1-15. (canceled)

16. A photoinitiator according to Formula (I): ##STR00036## wherein, R1
is selected from the group consisting of S1, --CN, --COR4, and a
functional group according to Formula (II): ##STR00037## R2 and R5 are
independently selected from the group consisting of hydrogen, an alkyl
group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl
group, and an aryl or heteroaryl group; S1 and S2 are independently
selected from the group consisting of hydrogen, an alkyl group, an
alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an
aryl or heteroaryl group, a halogen, --OH group, an alkoxy group, a thiol
group, a thioalkoxy group, an ester group, an amide group, an amine
group, and a carboxylic acid group; R3 is selected from the group
consisting of hydrogen, an alkyl group, an alkenyl group, an alkynyl
group, an aralkyl group, an alkaryl group, and an aryl or heteroaryl
group; R4 is selected from the group consisting of hydrogen, an alkyl
group, an alkenyl group, an alkynynl group, an aralkyl group, an alkaryl
group, an aryl or heteroaryl group, and W--R6; Q and X independently
represent O or N--R7; W, V, and Y independently represent O or N--R8; R6
and R8 independently are selected from the group consisting of hydrogen,
an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an
alkaryl group, and an aryl or heteroaryl group; R7 is selected from the
group consisting of hydrogen, an alkyl group, an alkenyl group, an
alkynyl group, an aralkyl group, an alkaryl group, an aryl or heteroaryl
group, and O--R9; R9 is selected from the group consisting of hydrogen,
an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an
alkaryl group, an aryl or heteroaryl group, and an acyl group; n and m
represent independently an integer from 1 to 3; with the proviso that at
least one of R1 to R3 comprises a branched, substituted or unsubstituted
alkyl, alkenyl, alkynyl, aralkyl, or alkaryl group; and substituted means
that a substituent is present containing at least one atom different from
carbon or hydrogen.

17. The photoinitiator according to claim 16, wherein R1 is selected from
the group consisting of hydrogen, --COR4, and a functional group
according to Formula (II).

18. The photoinitiator according to claim 16, wherein Q and X represent
O.

19. The photoinitiator according to claim 17, wherein Q and X represent
O.

20. The photoinitiator according to claim 16, wherein S1 and S2
represent hydrogen.

21. The photoinitiator according to claim 17, wherein S1 and S2
represent hydrogen.

22. The photoinitiator according to claim 18, wherein S1 and S2
represent hydrogen.

27. The photoinitiator according to claim 16, wherein the photoinitiator
is a diffusion hindered photoinitiator selected from the group consisting
of a polymerizable photoinitiator, a multifunctional photoinitiator, and
a polymeric or an oligomeric photoinitiator.

28. The photoinitiator according to claim 27, further comprising at least
one polymerizable ethylenically unsaturated group selected from the group
consisting of an acrylate, a methacrylate, an acrylamide a
methacrylamide, a styrene, a maleimide, a vinyl ester, a vinyl ether, an
allyl ether, and an allyl ester.

29. The photoinitiator according to claim 28, wherein at least one of R1
to R3 is substituted with the at least one polymerizable ethylenically
unsaturated group.

30. The photoinitiator according to claim 27, wherein one of the groups
selected from R1 to R3, S1, and S2 is linked to a polymer
selected from the group consisting of a star polymer, a dendritic
polymer, and a hyperbranched polymer.

31. The photoinitiator according to claim 30, wherein the hyperbranched
polymer is a polyether or a polyester.

32. The photoinitiator according to claim 16, wherein the photoinitiator
has a chemical structure according to Formula (III): ##STR00038##
wherein, R1, R3, S1, S2, n, and m are the same in as Formula
(I); L is a divalent linking group comprising 1 to 15 carbon atoms; and
R10 represents hydrogen or a C1 to C4 alkyl group.

33. The photoinitiator according to claim 32, wherein R10 represents
hydrogen or a methyl group.

34. A radiation curable composition comprising: a photoinitiator as
defined by claim 16.

35. A method for preparing a radiation curable composition as defined by
claim 16 comprising the steps of: a) providing a composition containing
monomers; and b) adding to the composition at least one photoinitiator
according to Formula (I) and at least one co-initiator selected from the
group consisting of an aliphatic tertiary amine and a dialkyl aniline
derivative.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a 371 National Stage Application of
PCT/EP2010/068940, filed Dec. 6, 2010. This application claims the
benefit of U.S. Provisional Application No. 61/267,468, filed Dec. 8,
2009, which is incorporated by reference herein in its entirety. In
addition, this application claims the benefit of European Application No.
09178164.1, filed Dec. 7, 2009, which is also incorporated by reference
herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a new class of photoinitiators,
especially suited for UV LED curable compositions.

[0004] 2. Description of the Related Art

[0005] Photoinitiators are frequently used in polymerizable compositions,
such as UV-curable inks, to initiate the polymerization of monomers when
exposed to UV radiation. Bathochromic photoinitiators, absorbing in the
region between 365 nm and 395 nm, are required to make full use of the
recent development of UV-LEDs with increasing power. Thioxanthones and
acyl phosphine oxides are photoinitiators absorbing in this spectral
region.

[0006] Thioxanthones are prone to yellowing upon exposure, thereby forming
degradation products with a limited stability. As a result, the original
yellowing shifts upon storage. Especially in imaging, e.g. inkjet
printing, this unstable yellowing behaviour makes control of the image
tone in the final image difficult. On top of that, certain applications,
predominantly packaging applications, prefer thioxanthone free radiation
curable compositions.

[0007] Acyl phosphine oxides, on the other hand, result in medium volatile
aldehyde type of degradation products, resulting in a background smell of
the printed image, which is unacceptable in packaging applications.

[0008] Therefore, there is an increasing demand for the development of new
photoinitiators, absorbing in the region between 365 nm and 395 nm,
having a stable yellowing behaviour without generating medium volatile
degradation products. Recent evolutions in bathochromic photoinitiators
are based on carbazole derivatives.

[0012] The carbazole initiators, disclosed in the prior art often require
multistep synthesis and are often still to hypsochromic to be cured by
LED curing. Therefore, there is still a need for easy accessible
photoinitiators exhibiting high curing speed upon LED exposure.

SUMMARY OF THE INVENTION

[0013] In order to overcome the problems described above, it has been
surprisingly found that a specific carbazole based photoinitiator
provided radiation curable compositions with high curing speed upon
exposure to UV radiation in the range between 365 nm and 395 nm.

[0014] According to preferred embodiments of the present invention, a
photoinitiator having a much more stable yellowing behaviour and without
generating medium volatile degradation products can be achieved, as
defined below.

[0015] Other features, elements, steps, characteristics and advantages of
the present invention will become more apparent from the following
detailed description of preferred embodiments of the present invention
hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The term "C.I." is used in disclosing the present application as an
abbreviation for Colour Index.

[0017] The term "alkyl" means all variants possible for each number of
carbon atoms in the alkyl group i.e. for three carbon atoms: n-propyl and
isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl;
for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl
and 2-methyl-butyl etc.

[0018] The term "substituted" in, for example substituted alkyl, means
that the substituent contains at least one atom different from carbon or
hydrogen. The substituent may be a single atom (e.g. a halogen) or a
group of atoms containing at least one atom different from carbon or
hydrogen (e.g. an acrylate group).

Photoinitiators

[0019] A photoinitiator according to a preferred embodiment of present
invention has as chemical structure the Formula (I):

##STR00001##

wherein, [0020] R1 selected from the group consisting of a group
according to S1,--CN, --COR4 and a functional group according to Formula
(II):

[0020] ##STR00002## [0021] R2 and R5 are independently selected from
the group consisting of hydrogen a substituted or unsubstituted alkyl
group, a substituted or unsubstituted alkenyl group, a substituted or
unsubstituted alkynyl group, a substituted or unsubstituted aralkyl
group, a substituted or unsubstituted alkaryl group and a substituted or
unsubstituted aryl or heteroaryl group; [0022] S1 and S2 are
independently selected from the group consisting of hydrogen, a
substituted or unsubstituted alkyl group, a substituted or unsubstituted
alkenyl group, a substituted or unsubstituted alkynyl group, a
substituted or unsubstituted aralkyl group, a substituted or
unsubstituted alkaryl group, a substituted or unsubstituted aryl, a
substituted or unsubstituted heteroaryl group, a halogen, OH, an alkoxy
group, a thiol group, a thioalkoxy group, an ester group, an amide group,
an amine group and a carboxylic acid group; [0023] R3 is selected from
the group consisting of hydrogen, a substituted or unsubstituted alkyl
group, a substituted or unsubstituted alkenyl group, a substituted or
unsubstituted alkynyl group, a substituted or unsubstituted aralkyl
group, a substituted or unsubstituted alkaryl group and a substituted or
unsubstituted aryl or heteroaryl group; [0024] R4 is selected from the
group consisting of a hydrogen a substituted or unsubstituted alkyl
group, a substituted or unsubstituted alkenyl group, a substituted or
unsubstituted alkynynl group, a substituted or unsubstituted aralkyl
group, a substituted or unsubstituted alkaryl group, a substituted or
unsubstituted aryl or heteroaryl group and W--R6; [0025] Q and X
independently represent O or N--R7; [0026] W, V and Y independently
represent O or N--R8; [0027] R6 and R8 independently are selected from
the group consisting of hydrogen a substituted or unsubstituted alkyl
group, a substituted or unsubstituted alkenyl group, a substituted or
unsubstituted alkynyl group, a substituted or unsubstituted aralkyl
group, a substituted or unsubstituted alkaryl group and a substituted or
unsubstituted aryl or heteroaryl group; [0028] R7 is selected from the
group consisting a hydrogen a substituted or unsubstituted alkyl group, a
substituted or unsubstituted alkenyl group, a substituted or
unsubstituted alkynyl group, a substituted or unsubstituted aralkyl
group, a substituted or unsubstituted alkaryl group, a substituted or
unsubstituted aryl or heteroaryl group and O--R9; [0029] R9 is selected
from the group consisting of hydrogen a substituted or unsubstituted
alkyl group, a substituted or unsubstituted alkenyl group, a substituted
or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl
group, a substituted or unsubstituted alkaryl group, a substituted or
unsubstituted aryl or heteroaryl group and an acyl group; [0030] n and m
independently represent an integer from 1 to 3; [0031] with the proviso
that at least one of R1 to R3 comprises a branched, substituted or
unsusbtituted alkyl, alkenyl, alkynyl, aralkyl or alkaryl group.

[0032] In a preferred embodiment, R1 is selected from the group consisting
of hydrogen, --COR4 and a functional group according to Formula (II). In
a further preferred embodiment, Q an X represent O.

[0033] In an even further preferred embodiment, S1 and S2
represent hydrogen. In the most preferred embodiment R3 represents a
branched alkyl group.

[0034] In a preferred embodiment, the photoinitiator, according to the
present invention is a diffusion hindered photoinitiator selected from
the group consisting of a polymerisable photoinitiator, a multifunctional
photoinitiator and a polymeric or an oligomeric photoinitiator. Preferred
polymeric photoinitiators, according to the present invention are
selected from the group consisting of star polymers, dendritic polymers
and hyperbranched polymers, polyesters and polyethers being particularly
preferred.

[0035] In a particularly preferred embodiment, the photoinitiator
according to the present invention is a photoinitiator according to
Formula (III):

##STR00003##

wherein, [0036] R1, R3, S1, S2, n and m are defined as for
Formula (I); [0037] L is a divalent linking group comprising 1 to 15
carbon atoms; [0038] R10 represents a hydrogen or a C1 to C4 alkyl group,
a hydrogen and a methyl group being particularly preferred and a hydrogen
being most preferred.

[0039] Suitable examples of photoinitiators according to Formula (I) are
given by Table 1, without being limited thereto.

[0040] For safety reasons, in particular for food packaging applications,
the photoinitiator is preferably a so-called diffusion hindered
photoinitiator. A diffusion hindered photoinitiator is a photoinitiator
which exhibits a much lower mobility in a cured layer of the curable
liquid or ink than a monofunctional photoinitiator, such as benzophenone.
Several methods can be used to lower the mobility of the photoinitiator.
One way is to increase the molecular weight of the photoinitiator so that
the diffusion speed is reduced, e.g. polymeric photoinitiators. Another
way is to increase its reactivity so that it is built into the
polymerizing network, e.g. multifunctional photoinitiators and
polymerizable photoinitiators. The diffusion hindered photoinitiator is
preferably selected from the group consisting of non-polymeric
multifunctional photoinitiators, polymeric photoinitiators and
polymerizable photoinitiators. Non-polymeric multifunctional
photoinitiators are considered to have a molecular weight between 300 and
900 Dalton. Non-polymerizable monofunctional photoinitiators with a
molecular weight in that range are not diffusion hindered
photoinitiators. Most preferably the diffusion hindered photoinitiator is
a polymerizable initiator.

[0041] Suitable polymerizable photoinitiators according to Formula (I) are
given in Table 1 by the photoinitiators INI-12 to INI-16.

[0042] A preferred amount of photoinitiator is 0.1-50 wt %, more
preferably 0.1-20 wt %, and most preferably 0.3-15 wt % of the total
weight of the curable pigment dispersion or ink.

Radiation Curable Compositions

[0043] The photoinitiators according to a preferred embodiment of the
present invention can be advantageously used in radiation curable
compositions to prevent unstable yellowing behaviour in an image upon
storage, e.g. an inkjet image.

[0044] In a preferred embodiment, the radiation curable composition is a
radiation curable inkjet ink, especially an inkjet ink curable by UV LEDs
emitting at in the spectral region of 365 nm to 395 nm. Due to their
compactness, UV LEDs can be built into inkjet printers more easily than
other UV light sources such as doped mercury lamps.

[0045] In a preferred embodiment, the radiation curable inkjet ink is part
of an inkjet ink set, preferably an inkjet ink set including two or more
inkjet inks in accordance with the invention. The radiation curable
inkjet ink form preferably part of a CMY(K) inkjet ink set. The CMY(K)
inkjet ink set may also be extended with extra inks such as red, green,
blue, violet and/or orange to further enlarge the colour gamut of the
image. The CMY(K) ink set may also be extended by the combination of full
density and light density inks of both colour inks and/or black inks to
improve the image quality by lowered graininess.

[0046] The inkjet ink can be advantageously used in an inkjet printing
method comprising the steps: [0047] a) providing a radiation curable
inkjet ink according to a preferred embodiment of the present invention;
and [0048] b) jetting the inkjet ink onto an ink-receiver.

[0049] The radiation curable composition according to a preferred
embodiment of the present invention may further also contain at least one
surfactant to control the homogenous spreading of the pigment dispersion
on a substrate. For an inkjet ink, the surfactant is important to control
the dot size of the ink droplet on a substrate.

[0050] There is no limitation on the viscosity of the radiation curable
composition, but the viscosity of a radiation curable inkjet ink is
preferably lower than 30 mPas, more preferably lower than 15 mPas, and
most preferably between 2 and 10 mPas at a shear rate of 100 s-1 and
a jetting temperature between 10 and 70° C.

[0051] The radiation curable composition according to the present
invention is preferably prepared according to a method comprising the
steps of: [0052] a) providing a composition containing monomers; [0053]
b) adding to said composition at least one co-initiator selected from the
group consisting of an aliphatic tertiary amine and a dialkyl aniline
derivative; and at least one photoinitiator according to Formula (I).

Co-Initiators

[0054] In order to increase the photosensitivity further, the radiation
curable composition contains a co-initiator. Suitable examples of
co-initiators can be categorized in 3 groups: [0055] (1) tertiary
aliphatic amines such as methyldiethanolamine, dimethylethanolamine,
triethanolamine, triethylamine and N-methylmorpholine; [0056] (2)
aromatic amines such as amylparadimethylaminobenzoate,
2-n-butoxyethyl-4-(dimethylamino) benzoate,
2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and
2-ethylhexyl-4-(dimethylamino)benzoate; and (3) (meth)acrylated amines
such as dialkylamino alkyl(meth)acrylates (e.g.,
diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates (e.g.,
N-morpholinoethyl-acrylate). The preferred co-initiators are
aminobenzoates.

[0057] The one or more co-initiators included into the radiation curable
composition according to a preferred embodiment of the present invention
are preferably diffusion hindered for safety reasons, in particular for
food packaging applications.

[0058] A diffusion hindered co-initiator is preferably selected from the
group consisting of non-polymeric multifunctional co-initiators,
oligomeric or polymeric co-initiators and polymerizable co-initiators.
More preferably the diffusion hindered co-initiator is selected from the
group consisting of polymeric co-initiators and polymerizable
co-initiators. Most preferably the diffusion hindered co-initiator is a
polymerizable co-initiator having at least one (meth)acrylate group, more
preferably having at least one acrylate group.

[0060] Preferred diffusion hindered co-initiators include a polymeric
co-initiator having a dendritic polymeric architecture, more preferably a
hyperbranched polymeric architecture. Preferred hyperbranched polymeric
co-initiators are those disclosed in US 2006014848 (AGFA) incorporated
herein as a specific reference.

[0061] The curable pigment dispersion or ink preferably comprises the
diffusion hindered co-initiator in an amount of 0.1 to 50 wt %, more
preferably in an amount of 0.5 to 25 wt %, most preferably in an amount
of 1 to 10 wt % of the total weight of the ink.

Monomers and Oligomers

[0062] The monomers and oligomers used in radiation curable compositions
and inks, especially for food packaging applications, are preferably
purified compounds having no or almost no impurities, more particularly
no toxic or carcinogenic impurities. The impurities are usually
derivative compounds obtained during synthesis of the polymerizable
compound. Sometimes, however, some compounds may be added deliberately to
pure polymerizable compounds in harmless amounts, for example,
polymerization inhibitors or stabilizers.

[0063] Any monomer or oligomer capable of free radical polymerization may
be used as polymerizable compound. A combination of monomers, oligomers
and/or prepolymers may also be used. The monomers, oligomers and/or
prepolymers may possess different degrees of functionality, and a mixture
including combinations of mono-, di-, tri-and higher functionality
monomers, oligomers and/or prepolymers may be used. The viscosity of the
radiation curable compositions and inks can be adjusted by varying the
ratio between the monomers and oligomers.

[0064] Particularly preferred monomers and oligomers are those listed in
[0106] to [0115] in EP 1911814 A (AGFA GRAPHICS) incorporated herein as a
specific reference.

[0065] A preferred class of monomers and oligomers are vinyl ether
acrylates such as those described in U.S. Pat. No. 6,310,115 (AGFA),
incorporated herein by reference. Particularly preferred compounds are
2-(2-vinyloxyethoxy)ethyl(meth)acrylate, most preferably the compound is
2-(2-vinyloxyethoxy)ethyl acrylate.

Colorants

[0066] Colorants used in the radiation curable compositions and inks may
be dyes, pigments or a combination thereof. Organic and/or inorganic
pigments may be used. The colorant is preferably a pigment or a polymeric
dye, most preferably a pigment.

[0069] Suitable pigments include mixed crystals of the above particular
preferred pigments. Mixed crystals are also referred to as solid
solutions. For example, under certain conditions different quinacridones
mix with each other to form solid solutions, which are quite different
from both physical mixtures of the compounds and from the compounds
themselves. In a solid solution, the molecules of the components enter
into the same crystal lattice, usually, but not always, that of one of
the components. The x-ray diffraction pattern of the resulting
crystalline solid is characteristic of that solid and can be clearly
differentiated from the pattern of a physical mixture of the same
components in the same proportion. In such physical mixtures, the x-ray
pattern of each of the components can be distinguished, and the
disappearance of many of these lines is one of the criteria of the
formation of solid solutions. A commercially available example is
Cinquasia Magenta RT-355-D from Ciba Specialty Chemicals.

[0070] Also mixtures of pigments may be used in the UV curable inks. For
some inkjet applications, a neutral black inkjet ink is preferred and can
be obtained, for example, by mixing a black pigment and a cyan pigment
into the ink. The inkjet application may also require one or more spot
colours, for example for packaging inkjet printing or textile inkjet
printing. Silver and gold are often desired colours for inkjet poster
printing and point-of-sales displays.

[0072] Pigment particles in inkjet inks should be sufficiently small to
permit free flow of the ink through the inkjet-printing device,
especially at the ejecting nozzles. It is also desirable to use small
particles for maximum colour strength and to slow down sedimentation.

[0073] The numeric average pigment particle size is preferably between
0.050 and 1 μm, more preferably between 0.070 and 0.300 μm and
particularly preferably between 0.080 and 0.200 μm. Most preferably,
the numeric average pigment particle size is no larger than 0.150 μm.
An average particle size smaller than 0.050 μm is less desirable for
decreased light-fastness, but mainly also because very small pigment
particles or individual pigment molecules thereof may still be extracted
in food packaging applications. The average particle size of pigment
particles is determined with a Nicomp 30 Submicron Particle Analyzer
based upon the principle of dynamic light scattering. The ink is diluted
with ethyl acetate to a pigment concentration of 0.002 wt %.

[0074] However for a white UV curable ink, the numeric average particle
diameter of the white pigment is preferably from 50 to 500 nm, more
preferably from 150 to 400 nm, and most preferably from 200 to 350 nm.
Sufficient hiding power cannot be obtained when the average diameter is
less than 50 nm, and the storage ability and the jet-out suitability of
the ink tend to be degraded when the average diameter exceeds 500 nm. The
determination of the numeric average particle diameter is best performed
by photon correlation spectroscopy at a wavelength of 633 nm with a 4 mW
HeNe laser on a diluted sample of the pigmented inkjet ink. A suitable
particle size analyzer used was a MALVERN® nano-S available from
Goffin-Meyvis. A sample can, for example, be prepared by addition of one
drop of ink to a cuvet containing 1.5 mL ethyl acetate and mixed until a
homogenous sample was obtained. The measured particle size is the average
value of 3 consecutive measurements consisting of 6 runs of 20 seconds.

[0075] Suitable white pigments are given by Table 2 in [0116] of WO
2008/074548 (AGFA GRAPHICS). The white pigment is preferably a pigment
with a refractive index greater than 1.60. The white pigments may be
employed singly or in combination. Preferably titanium dioxide is used as
pigment with a refractive index greater than 1.60. Suitable titanium
dioxide pigments are those disclosed in [0117] and in [0118] of WO
2008/074548 (AGFA GRAPHICS).

[0076] The pigments are present in the range of 0.01 to 10% by weight,
preferably in the range of 0.1 to 5% by weight, each based on the total
weight of UV curable ink. For white UV curable inks, the white pigment is
preferably present in an amount of 3% to 30% by weight of the ink
composition, and more preferably 5% to 25%. An amount of less than 3% by
weight cannot achieve sufficient covering power and usually exhibits very
poor storage stability and ejection property.

[0077] Generally pigments are stabilized in the dispersion medium by
dispersing agents, such as polymeric dispersants. However, the surface of
the pigments can be modified to obtain so-called "self-dispersible" or
"self-dispersing" pigments, i.e. pigments that are dispersible in the
dispersion medium without dispersants.

Dispersants

[0078] The dispersant is preferably a polymeric dispersant. Typical
polymeric dispersants are copolymers of two monomers but may contain
three, four, five or even more monomers. The properties of polymeric
dispersants depend on both the nature of the monomers and their
distribution in the polymer. Suitable copolymeric dispersants have the
following polymer compositions:

[0082] block copolymers (e.g. monomers A and B polymerized into
AAAAABBBBBB) wherein the block length of each of the blocks (2, 3, 4, 5
or even more) is important for the dispersion capability of the polymeric
dispersant;

[0083] graft copolymers (graft copolymers consist of a polymeric backbone
with polymeric side chains attached to the backbone); and

[0101] The polymeric dispersant is preferably used in an amount of 2 to
600 wt %, more preferably 5 to 200 wt % based on the weight of the
pigment.

Dispersion Synergists

[0102] A dispersion synergist usually consists of an anionic part and a
cationic part. The anionic part of the dispersion synergist exhibiting a
certain molecular similarity with the colour pigment and the cationic
part of the dispersion synergist consists of one or more protons and/or
cations to compensate the charge of the anionic part of the dispersion
synergist.

[0103] The synergist is preferably added in a smaller amount than the
polymeric dispersant(s). The ratio of polymeric dispersant/dispersion
synergist depends upon the pigment and should be determined
experimentally. Typically the ratio wt % polymeric dispersant/wt %
dispersion synergist is selected between 2:1 to 100:1, preferably between
2:1 and 20:1.

[0104] Suitable dispersion synergists that are commercially available
include SOLSPERSE® 5000 and SOLSPERSE® 22000 from NOVEON.

[0105] Particular preferred pigments for the magenta ink used are a
diketopyrrolo-pyrrole pigment or a quinacridone pigment. Suitable
dispersion synergists include those disclosed in EP 1790698 A (AGFA
GRAPHICS), EP 1790696 A (AGFA GRAPHICS), WO 2007/060255 (AGFA GRAPHICS)
and EP 1790695 A (AGFA GRAPHICS).

[0106] In dispersing C.I. Pigment Blue 15:3, the use of a sulfonated
Cu-phthalocyanine dispersion synergist, e.g. SOLSPERSE® 5000 from
NOVEON is preferred. Suitable dispersion synergists for yellow inkjet
inks include those disclosed in EP 1790697 A (AGFA GRAPHICS).

Surfactants

[0107] The radiation curable compositions and inks may contain a
surfactant. The surfactant(s) can be anionic, cationic, non-ionic, or
zwitter-ionic and are usually added in a total quantity less than 10 wt %
based on the total weight of the radiation curable composition or ink and
particularly in a total less than 5 wt % based on the total weight of the
radiation curable composition or ink.

[0108] Suitable surfactants include those disclosed in paragraphs [0283]
to [0291] of WO 2008/074548 (AGFA GRAPHICS) incorporated herein as a
specific reference.

[0112] Since excessive addition of these polymerization inhibitors may
lower the curing speed, it is preferred that the amount capable of
preventing polymerization is determined prior to blending. The amount of
a polymerization inhibitor is preferably lower than 5 wt %, more
preferably lower than 3 wt % of the total radiation curable composition
or ink.

Preparation of Curable Inks

[0113] The average particle size and distribution of a colour pigment is
an important feature for inkjet inks. The inkjet ink may be prepared by
precipitating or milling the pigment in the dispersion medium in the
presence of the dispersant.

[0114] Mixing apparatuses may include a pressure kneader, an open kneader,
a planetary mixer, a dissolver, and a Dalton Universal Mixer. Suitable
milling and dispersion apparatuses are a ball mill, a pearl mill, a
colloid mill, a high-speed disperser, double rollers, a bead mill, a
paint conditioner, and triple rollers. The dispersions may also be
prepared using ultrasonic energy.

[0115] Many different types of materials may be used as milling media,
such as glasses, ceramics, metals, and plastics. In a preferred
embodiment, the grinding media can comprise particles, preferably
substantially spherical in shape, e.g. beads consisting essentially of a
polymeric resin or yttrium stabilized zirconium oxide beads.

[0116] In the process of mixing, milling and dispersion, each process is
performed with cooling to prevent build up of heat, and as much as
possible under light conditions in which actinic radiation has been
substantially excluded.

[0117] The inkjet ink may contain more than one pigment, and may be
prepared using separate dispersions for each pigment, or alternatively
several pigments may be mixed and co-milled in preparing the dispersion.

[0118] The dispersion process can be carried out in a continuous, batch or
semi-batch mode.

[0119] The preferred amounts and ratios of the ingredients of the mill
grind will vary widely depending upon the specific materials and the
intended applications. The contents of the milling mixture comprise the
mill grind and the milling media. The mill grind comprises pigment,
polymeric dispersant and a liquid carrier. For inkjet inks, the pigment
is usually present in the mill grind at 1 to 50 wt %, excluding the
milling media. The weight ratio of pigment over polymeric dispersant is
20:1 to 1:2.

[0120] The milling time can vary widely and depends upon the pigment,
mechanical means and residence conditions selected, the initial and
desired final particle size, etc. In a preferred embodiment of the
present invention pigment dispersions with an average particle size of
less than 100 nm may be prepared.

[0121] After milling is completed, the milling media is separated from the
milled particulate product (in either a dry or liquid dispersion form)
using conventional separation techniques, such as by filtration, sieving
through a mesh screen, and the like. Often the sieve is built into the
mill, e.g. for a bead mill. The milled pigment concentrate is preferably
separated from the milling media by filtration.

[0122] In general it is desirable to make the inkjet inks in the form of a
concentrated mill grind, which is subsequently diluted to the appropriate
concentration for use in the inkjet printing system. This technique
permits preparation of a greater quantity of pigmented ink from the
equipment. By dilution, the inkjet ink is adjusted to the desired
viscosity, surface tension, colour, hue, saturation density, and print
area coverage for the particular application.

EXAMPLES

Materials

[0123] All materials used in the following examples were readily available
from standard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS
(Belgium) unless otherwise specified. The water used was deionized water.

[0124] DPGDA is dipropyleneglycoldiacrylate from SARTOMER. TMPTA is
trimethylolpropane triacrylate available as SARTOMER® SR351 from
SARTOMER. VEEA is 2-(vinylethoxy)ethyl acrylate, a difunctional monomer
available from NIPPON SHOKUBAI, Japan.

[0125] EPD is ethyl 4-dimethylaminobenzoate, available under the trade
name of Genocure® EPD from RAHN AG. IC127 is an abbreviation used for
IRGACURE® 127, supplied by Ciba Specialty Chemicals:

##STR00024##

IC907 is an abbreviation used for IRGACURE® 907 is
2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one, a
photoinitiator available from CIBA SPECIALTY CHEMICALS. IC379 is an
abbreviation used for IRGACURE® 379 is a photoinitiator available from
CIBA SPECIALTY having as chemical structure:

##STR00025## [0126] ITX is an abbreviation used for DAROCUR® ITX, an
isomeric mixture of 2- and 4-isopropylthioxanthone from CIBA SPECIALTY
CHEMICALS. [0127] TPO is an abbreviation used for
2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide available under the trade
name DAROCUR® TPO from CIBA SPECIALTY CHEMICALS. [0128] GENORAD® 16
is a polymerization inhibitor from RAHN AG. [0129] GENOSOL is a 50wt %
solution of GENORAD® 16 in DPGDA.

[0130] PB15:4 is an abbreviation used for HOSTAPERM® Blue P-BFS, a cyan
pigment (C.I. Pigment Blue 15:4) available from CLARIANT. [0131] DB162 is
an abbreviation used for the polymeric dispersant DISPERBYK® 162
available from BYK CHEMIE GMBH whereof the solvent mixture of
2-methoxy-1-methylethylacetate, xylene and n-butylacetate was removed.
[0132] DB162sol is a 30 wt % solution of DB162 in DPGDA. [0133] S35000 is
an abbreviation used for SOLSPERSE® 35000, a
polyethyleneimine-polyester hyperdispersant from NOVEON. [0134] S35000SOL
is a 40 wt % solution of 535000 in DPGDA. [0135] BYK® UV3510 is a
polyether modified polydimethylsiloxane wetting agent available from BYK
CHEMIE GMBH.

Measurement Methods

1. Curing Speed

[0136] The curing speed on a Fusion DRSE-120 conveyor was defined as the
percentage of the maximum output of the lamp needed to cure the samples.
The lower the number the higher curing speed. A sample was considered as
fully cured at the moment scratching with a Q-tip caused no visual
damage.

[0137] A percentage of more then 100% of the maximum output of the lamp
means that the speed of the conveyor belt had to be reduced to get the
sample fully cured at the maximum output of the lamp. The higher the
percentage, the more the belt had to be slowed down. A curing speed of
160% means a belt speed of 12.5 m/min at the maximum output of the lamp.
A percentage between 150% and 200% is considered as at the edge of
practical use. A percentage above 200% is considered out of the range for
practical use and no higher percentages are measured.

2. Curing Degree

[0138] The curing degree was tested on a coating immediately after curing
with UV light. The cured coating is rubbed with the means of a Q-tip.
When the surface is not damaged, the coating is fully cured. When some of
the cured coating can be damaged, the coating is only partly cured. When
the whole cured coating is damaged, the coating is not cured.

3. Average Particle Size

[0139] The particle size of pigment particles in an inkjet ink was
determined by photon correlation spectroscopy at a wavelength of 633 nm
with a 4 mW HeNe laser on a diluted sample of the pigmented inkjet ink.
The particle size analyzer used was a MALVERN® nano-S available from
Goffin-Meyvis.

[0140] The sample was prepared by addition of one drop of ink to a cuvette
containing 1.5 mL ethyl acetate and mixed until a homogenous sample was
obtained. The measured particle size is the average value of 3
consecutive measurements consisting of 6 runs of 20 seconds. For good ink
jet characteristics (jetting characteristics and print quality) the
average particle size of the dispersed particles is below 200 nm,
preferably between 70 and 150 nm.

4. Image Tone

[0141] Printed or coated samples were measured with a spectrophotometer
(GRETAG SPM, manufactured by GRETAG INC.) to determine the coordinates of
the L*a*b* colours system of the colour difference indication method
specified in CIE (Commission International de l'Eclairage). In this case,
the measurement was carried out under conditions of light source D50,
provision of no light source filter, absolute white as reference white,
and angle of visibility 2°.

5. Degree of Conversion

[0142] The degree of conversion, i.e. the percentage of converted
functional groups, may be determined by for example RT-FTIR (Real-Time
Fourier Transform Infra-Red Spectroscopy).

[0143] From a radiation curable composition, an FTIR-spectrum, using a
micro-ATR method on a BIO-RAD FTS-7 spectrometer, equipped with a split
pea module from Harrick, was taken before curing, by applying a drop of
ink on the split pea module. A second sample of the radiation curable
composition was coated on a PGA-paper, using a bar coater and a 10 μm
wired bar. The coated sample was mounted on a belt, transporting the
samples under a Phoseon 4W 395 nm LED at a speed specified in the
examples.

[0144] After coating and curing the radiation curable composition as
described above, a second FTIR-spectrum was taken from each coated and
cured sample under the same conditions. The change in peak height at 810
cm-1, corresponding to a C--H vibration on the double bonds was
measured relative to the C═O-stretching vibration at 1728 cm-1,
which was used as an internal reference and the following two ratios were
determined:

ratiocuring=I810((curing)/I1728((curing)

ratioref=I810(ref)/I1728(ref)

wherein I corresponds to the respective peak heights. It was assumed that
the ester function remained unchanged during curing. The curing
percentage was calculated as follows:

Curing %=100-(ratiocuring/ratioref)*100

[0145] A full cure is defined as a degree of conversion wherein the
increase in the percentage of converted functional groups, with increased
exposure to radiation (time and/or dose), is negligible. A full cure
corresponds with a conversion percentage that is within 10%, preferably
5%, from the maximum conversion percentage defined by the horizontal
asymptote in the RT-FTIR graph (percentage conversion versus curing
energy or curing time).

Example 1

[0146] This example illustrates the simplicity of the method for preparing
a photoinitiator according to a preferred embodiment of the present
invention.

Photoinitiator INI-1

[0147] First, 9-(2-Ethyl-hexyl)-9H-carbazole was synthesized according to
the following synthesis scheme:

##STR00026##

To a pale brown solution of 9H-carbazole (66.9 g, 0.4 mol),
3-bromomethyl-heptane (125.5 g, 0.65 mol) and tetrabutylammonium
hydrogensulfate (37.3 g, 0.11 mol) in acetone (650 ml), potassium
hydroxide (86%) (49.6 g, 0.76 mol) was added in portions. The reaction
mixture was heated to reflux temperature and stirred for about 16 hours.
The inorganic residues were removed by filtration and the solvent was
evaporated under reduced pressure. The residual oil was dissolved in
methyl-tert-butylether (500 ml) and extracted with distilled water (500
ml). The aqueous layer was extracted with dichloromethane (350 ml).

[0148] The pooled organic fractions were dried over MgSO4 and the
solvent was evaporated under reduced pressure to obtain a brown oil. The
crude 9-(2-ethyl-hexyl)-9H-carbazole was purified on a Merck SVP D40
Column using n-hexane as eluent. Evaporation of the pooled fractions
yielded 85 g of 9-(2-Ethyl-hexyl)-9H-carbazole.

[0149] Then,
2[6-Ethoxyoxalyl-9-(2-ethyl-hexyl)-9H-carbazol-3-yl]-oxo-acetic acid
ethyl ester was synthesized according to the following synthesis scheme:

##STR00027##

[0150] To a solution of 9-(2-ethyl-hexyl)-9H-carbazole (7.0 g, 0.025 mol)
in dichloromethane (30 ml), ethyloxalyl chloride (7.2 g, 0.0525 mol) was
added and stirred for 15 minutes at room temperature. The reaction
mixture was cooled to -5° C. and aluminium chloride (7.3 g, 0.055
mol) was added in portions while the temperature was maintained below
0° C. The reaction mixture was allowed to stir at room temperature
for 1.5 hours. The reaction mixture was poured into ice (200 g) and
diluted with dichloromethane (100 ml). The organic layer was separated
and extracted 5 times with distilled water (150 ml). The organic layer
was dried over MgSO4, filtered and the solvent was removed under
reduced pressure. The residual oil was purified on a Prochrom LC80 Column
using dichloromethane as eluent. Evaporation of the pooled fractions
yielded 4.3 g of INI-1. (Rf:0.25, eluent 100% methylene chloride,
Merck Kieselgel 60 F254)

Photoinitiator INI-2

[0151] First, [9-(2-ethyl-hexyl)-9H-carbazol-3-yl]-phenyl-methanone was
synthesized according to the following synthesis scheme:

##STR00028##

To a solution of 9-(2-ethyl-hexyl)-9H-carbazole (7.0 g, 0.025 mol) in
dichloromethane (30 ml), benzoyl chloride (4.5 g, 0.0525 mol) was added
and stirred for 15 minutes at room temperature. Aluminium chloride (7.3
g, 0.055 mol) was added in portions while the temperature was maintained
below 30° C. The reaction mixture was allowed to stir at room
temperature for 15 hours. The reaction mixture was poured into ice (150
g) and distilled water (100 ml) and diluted with dichloromethane (200
ml). The organic layer was separated and washed with a saturated solution
of sodium bicarbonate (250 ml) and a saturated solution of sodium
chloride (250 ml). The organic layer was dried over MgSO4, filtered
and the solvent was removed under reduced pressure. The residual solid
was purified on a Prochrom LC80 Column using ethyl acetate/n-hexane
(20/80) as eluent. TLC shows a yellow fluorescent product with Rf-value
of 0.48 in dichloromethane as eluent. Evaporation of the pooled fractions
yielded 1.5 g of [9-(2-ethyl-hexyl)-9H-carbazol-3-yl]-phenyl-methanone.

[0152] Then, [6-benzoyl-9-(2-ethyl-hexyl)-9H-carbazol-3-yl]-oxo-acetic
acid ethyl ester was synthesized according to the following synthesis
scheme:

##STR00029##

To a solution of [9-(2-ethyl-hexyl)-9H-carbazole-3-yl]-phenyl-methanone
(4.4 g, 0.011 mol) in dichloromethane (40 ml), ethyloxalyl chloride (4.3
g, 0.03135 mol) was added and stirred for 15 minutes at room temperature.
Aluminium chloride (4.2 g, 0.03135 mol) was added in portions while the
temperature was maintained below 30° C. The reaction mixture was
allowed to stir at room temperature for 48 hours. The reaction mixture
was poured into ice (100 g) and distilled water (50 ml) and diluted with
dichloromethane (60 ml). The organic layer was separated, dried over
MgSO4 and filtered. The solvent was removed under reduced pressure.
The residue was purified on a Prochrom LC80 Column using dichloromethane
as eluent. Evaporation of the pooled fractions yielded 3.6 g of INI-2
(Rf:0.28, eluent:100% methylene chloride, Merck Kieselgel 60
F254).

Photoinitiators INI-3

[0153] [9-(2-ethyl-hexyl)-9H-carbazol-3-yl]-oxo-acetic acid ethyl ester
was synthesized according to the following synthesis scheme:

##STR00030##

[0154] To a solution of 9-(2-ethyl-hexyl)-9H-carbazole (7.0 g, 0.025 mol)
in dichloromethane (30 ml), chloro-oxo-acetic acid ethyl ester (7.2 g,
0.0525 mol) was added and stirred for 15 minutes at room temperature. The
reaction mixture was cooled to -5° C. and aluminium chloride (7.3
g, 0.055 mol) was added in portions while the temperature was maintained
below 0° C. The reaction mixture was allowed to stir at room
temperature for 90 minutes. The reaction mixture was poured into ice (200
g) and diluted with dichloromethane (100 ml). The organic layer was
separated and washed five times with distilled water (150 ml). The
organic layer was dried over MgSO4 and filtered. The solvent was
removed under reduced pressure. The residual oil was purified on a
Prochrom LC80 using dichloromethane as eluent. Evaporation of pooled
fractions yielded 3.8 g of INI-3 (Rf:0.6, eluent:100% methylene
chloride, Merck Kieselgel 60 F254).

Photoinitiators INI-6 and INI-20

[0155] [9-6-(2-ethyl-hexyloxyoxalyl)-9H-carbazol-3-yl)-oxo-acetic acid
2-ethyl-hexyl ester was synthesized according to the following synthesis
scheme:

##STR00031##

[0156] The Friedel-Crafts-acylation: 60 g (0.307 mol) of 9-ethyl-carbazole
(supplied by Aldrich) was dissolved in 250 ml methylene chloride. 92.3 g
(0.676 mol) ethyl-oxalyl chloride was added. 90 g (0.676 mol) aluminium
chloride was added portion wise, while maintaining the temperature below
35° C. Upon complete addition, the reaction mixture was cooled to
-10° C. and the reaction was allowed to continue for 24 hours. The
reaction mixture became difficult to stir. 350 ml ethyl acetate was added
at room temperature and the reaction mixture was stirred for 16 hours.
The reaction mixture was poured into 360 g ice and an additional 360 ml
ethyl acetate was added. The organic fraction was isolated, extracted
with brine and twice with 200 ml of a saturated NaHCO3 solution. The
intermediate diester precipitated from the organic fraction and was
isolated by filtration. The filtrate was evaporated under reduced
pressure and the residue was treated with toluene, yielding a second crop
of the diester. The fractions were pooled and 52.8 g (43.6%) of the
diester was isolated (Rf:0.23, eluent 70/30 hexane/ethyl acetate on
Merck Kieselgel 60 F254.). The intermediate was used without further
purification.

[0157] The hydrolysis of the esters:

[0158] 42.8 g (0.108 mol) of the diester was dissolved in 150 ml ethanol.
13 g (0.322 mol) NaOH was added and the reaction mixture was heated to
60° C. The reaction was allowed to continue for one hour at
60° C. 100 ml water was added to the reaction mixture and the
mixture was acidified to pH=1, using a 6N hydrochloric acid solution. The
dicarboxylic acid precipitated from the medium, was isolated by
filtration and dried. 18.7 g (52%) of the dicarboxylic acid was isolated.
LCMS analysis indicated a purity of 95%. The dicarboxylic acid was used
without further purification.

[0159] The CDI-coupling:

[0160] 7.5 g (22 mmol) of the dicarboxylic acid was dissolved in 20 ml
dimethyl acetamide. 6.9 g (41 mmol) CDI was added. The temperature rose
to 35° C. and the reaction was allowed to continue for 4 hours at
80° C. 5.8 g (44 mmol) 2-ethy-hexyl alcohol was added and the
reaction was allowed to continue for one hour at 80° C. After one
hour, an additional 5.8 g (44 mmol) 2-ethyl-hexyl alcohol was added and
the reaction was allowed to continue at 80° C. for 16 hours. After
cooling down to room temperature, 150 ml methyl tert.butyl ether and 100
ml water were added to the reaction mixture. The aqueous layer was
extracted three times with 100 ml methyl tert.butyl ether. The organic
fractions were pooled, dried over MgSO4 and evaporated under reduced
pressure. INI-6 was isolated by preparative column chromatography on a
Prochrom LC80 column, using Kromasil Si 60A 10 μm and a gradient
elution from 100% methylene chloride to methylene chloride/ethyl acetate
94/6 at a flow rate of 150 ml/min. (Rf:0.35, eluent MeOH/NaCl 90/10,
Partisil KC18F).

[0167] Acrylic acid
4-(3-(2-[6-benzoyl-9-(2-ethyl-hexyl)-9H-carbazol-3-yl]-2-oxo-acetoxy)-2-h-
ydroxy-propoxy)-butyl ester was synthesized according to the following
synthesis scheme:

##STR00033##

[0168] Hydrolysis of INI-2:

[0169] 2.2 g (4.6 mmol) of INI-2 was dissolved in 20 ml ethanol. The
reaction mixture was heated to 50° C. and 0.46 ml of a 10 N NaOH
solution (4.6 mmol) was added. The reaction was allowed to continue for
three hours at 50° C. The solvent was removed under reduced
pressure and the residue was dissolved in 10 ml water. The mixture was
acidified with a 6N hydrochloric acid solution. The mixture was extracted
with 20 ml methyl tert.butyl ether. The organic fraction was isolated,
dried over MgSO4 and evaporated under reduced pressure. 2.1 g of the
intermediate carboxylic acid was isolated. The intermediate was used
without further purification.

[0170] Reaction with the Epoxy-Acrylate:

[0171] 2 g (4.4 mmol) of the intermediate carboxylic acid was dissolved in
20 ml acetonitrile. 10 mg BHT and 0.14 g (0.44 mmol) tetrabutylammonium
bromide were added and the mixture was heated to reflux. 0.9 g (4.4 mmol)
of 4-(glycidyloxy)butyl acrylate was added and the reaction was allowed
to continue for 16 hours at reflux temperature. The solvent was removed
under reduced pressure and INI-16 was purified by preparative column
chromatography.

[0172] Photoinitiator INI-8

[0173] INI-8 was synthesized according to the following synthesis scheme:

##STR00034##

[0174] Synthesis of the Oximes:

[0175] 3.8 g (7.9 mmol) of INI-1 was dissolved in 34 ml pyridine and 22 ml
ethanol. 1.2 g (16.7 mmol) hydroxyl amine chlorohydrate was added and the
reaction mixture was refluxed for 16 hours. An additional 1.2 g (16.7
mmol) hydroxyl amine chlorohydrate was added and the reaction was allowed
to continue for an additional three hours at reflux temperature. The
solvent was evaporated under reduced pressure and the residue was
dissolved in 100 ml methylene chloride. The mixture was extracted with
100 ml 1 N hydrochloric acid and twice with 100 ml water. The organic
fraction was isolated, dried over MgSO4 and evaporated under reduced
pressure. The intermediate oxime was isolated by preparative column
chromatography on a Prochrom LC80 column, using Kromasil Si 60A 10 μm
and a gradient elution from n.-hexane/ethyl acetate 70/30 to
n.-hexane/ethyl acetate 50/50 at a flow rate of 150 ml/min. 1.4 g of the
intermediate bis-oxime was isolated (Rf:0.5, eluent n.-hexane/ethyl
acetate 50/50, Merck Kieselgel 60 F254).

[0178] This example illustrates the need for introducing a branched
substituent on the nitrogen of the carbazole photoinitiators according to
a preferred embodiment of the present invention and further illustrates
their high photoreactivity and excellent yellowing behaviour.

Preparation of Concentrated Pigment Dispersions DISP-1 and DISP-2

[0179] Concentrated Pigment Dispersions DISP-1:

[0180] 45,000 g of DB162sol and 450 g of GENOSOL were dissolved in 31,050
g of DPGDA in a vessel of 125 L using a DISPERLUX® disperser (from
DISPERLUX S.A.R.L., Luxembourg). 13,500 g of cyan pigment PB15:4 was
added to the solution and stirred for 30 minutes. The vessel was then
connected to a Netzsch LMZ10 mill having an internal volume of 10 L
filled for 52% with 0.4 mm yttrium stabilized zirconia beads ("high wear
resistant zirconia grinding media" from TOSOH Co.). The mixture was
circulated over the mill for 7 hours and 45 minutes at a flow rate of
about 2 L per minute and a rotation speed in the mill of about 15 m/s.
During the complete milling procedure the content in the mill was cooled
to a temperature of 42° C. After milling, the concentrated pigment
dispersion DISP-1 was discharged into another 125 L vessel. The resulting
concentrated pigment dispersion DISP-1 according to Table 2 exhibited an
average particle size of 110 nm.

[0182] 15,000 g of S35000sol and 300 g of GENOSOL were dissolved in 8,850
g of DPGDA in a vessel of 60 L. 6,000 g of cyan pigment PB15:4 was added
to the solution and stirred for 30 minutes using a DISPERLUX®
disperser (from DISPERLUX S.A.R.L., Luxembourg). The vessel was then
connected to a Bachofen DYNOMILL ECM POLY mill having an internal volume
of 8.2 L filled for 42% with 0.4 mm yttrium stabilized zirconia beads
("high wear resistant zirconia grinding media" from TOSOH Co.). The
mixture was circulated over the mill for 1 hour and 50 minutes at a flow
rate of about 5 L per minute and a rotation speed in the mill of about 15
m/s. During the complete milling procedure the content of the mill was
cooled to a temperature of 54° C. The concentrated pigment
dispersion DISP-2 was discharged into another 60 L vessel. The resulting
concentrated pigment dispersion DISP-2 according to Table 3 exhibited an
average particle size of 119 nm.

[0184] (6-Ethoxyoxalyl-9-ethyl-9H-carbazol-3-yl)-oxo-acetic acid ethyl
ester was synthesized according to the following synthesis scheme:

##STR00035##

[0185] To a dark brown solution of 9-ethylcarbazole (14.6 g, 0.075 mol) in
dichloromethane (100 ml), ethyloxalyl chloride (22.5 g, 0.0165 mol) was
added and stirred for 15 minutes at room temperature. Aluminium chloride
(22.0 g, 0.0165 mol) was added in portions while the temperature was
maintained below 30° C. The reaction mixture was allowed to stir
at room temperature for 24 hours. Ethyl acetate was added (150 ml), and
the mixture was again stirred for 24 hours at room temperature. The
reaction mixture was diluted with ethyl acetate (600 ml) and poured into
ice (250 g) and distilled water (250 ml).

[0186] After stirring for 1 hour, the organic layer was separated and
washed once with a saturated solution of sodium bicarbonate (150 ml) and
once with a saturated solution of sodium chloride (300 ml). The organic
layer was separated, dried on MgSO4 and the solvent was evaporated
under reduced pressure to obtain a yellow solid. The residue was
dissolved in dichloromethane (36 ml) and n-hexane (100 ml) was added. The
crude (6-ethoxyoxalyl-9-ethyl-9H-carbazol-3-yl)-oxo-acetic acid ethyl
ester precipitated from the medium and was isolated by filtration. The
crude (6-ethoxyoxalyl-9-ethyl-9H-carbazol-3-yl)-oxo-acetic acid ethyl
ester was purified on a Merck SVP D40 Column using
n-hexane/dichloromethane (50/50) as eluent. After evaporation of the
pooled fractions, the residue was dissolved in dichloromethane and
n-hexane was added. Crystallization provided 7.4 g of COMPINI-1.
(Rf:0.25, eluent:n.-hexane/ethyl acetate 70/30, Merck Kieselgel 60
F254)

Preparation of Radiation Curable Compositions

[0187] The comparative radiation curable compositions COMP-1 to COMP-4 and
the inventive radiation curable compositions INV-1 to INV-4 were prepared
according to Table 4. The weight % (wt %) was based on the total weight
of the radiation curable compositions.

[0188] The comparative radiation curable compositions COMP-3 and COMP-4
could not be formulated due to insolubility of the carbazole based
initiator COMPINI-1. Introducing a branched alkyl chain as in INI-1 and
INI-3 resulted in initiators having a high compatibility with radiation
curable compositions.

[0189] The comparative radiation curable compositions COMP-1 and COMP-2
and inventive radiation curable compositions INV-1 to INV-4 were coated
on PGA-paper, using a bar coater and a 10 μm wired bar. The coated
samples were mounted on a belt, transporting the samples under a Phoseon
4W 395 nm LED. The number of passes at a given belt speed to completely
cure the samples was determined. The Q-tip method was used to determine
complete cure. The results are summarized in Table 5.

[0190] The radiation curable compositions INV-1 to INV-4 and COMP-2 were
cured on a Fusion DRSE-120 conveyor equipped with Fusion VPS/1600 lamp at
20m/min at full power of the lamp. A second sample was cured with a
Phoseon 4W 395 nm LED, passing the sample 4 times under the LED at a
speed of 5 m/min. The stability of the image tone under both curing
conditions was quantified by measuring the shift in b-value between the
freshly printed sample and the sample stored for 7 days at ambient
temperature. The results are summarized in Table 6.

[0191] From Table 6, it becomes apparent that the radiation curable
compositions according to preferred embodiments of the present invention
have a significantly more stable yellowing behaviour in comparison with
thioxanthones.

Example 3

[0192] This example illustrates the high curing speed of radiation curable
inkjet inks according to a preferred embodiment of the present invention.

Preparation of Radiation Curable Inkjet Inks

[0193] The comparative radiation curable compositions COMP-5 and COMP-6
and the inventive radiation curable compositions INV-5 to INV-7 were
prepared according to Table 7. The weight % (wt %) was based on the total
weight of the radiation curable compositions.

[0194] The compositions INV-5 to INV-7 and COMP-5 and COMP-6 were cured on
a Fusion DRSE-120 conveyor equipped with Fusion VPS/1600 lamp (D-bulb) at
40m/min at full power of the lamp. The degree of curing was evaluated
using a Q-tip. A sample was considered as fully cured at the moment
scratching with a Q-tip caused no visual damage. The results are
summarized in Table 8.

[0195] From Table 8, it becomes apparent that the radiation curable
compositions according to preferred embodiments of the present invention
are highly sensitive.

Example 4

[0196] This example illustrates that the radiation curable inkjet inks
according to a preferred embodiment of the present invention exhibit a
high curing speed and do not have the undesirable yellowing behaviour of
ITX.

Preparation of Radiation Curable Inkjet Inks

[0197] The comparative radiation curable compositions COMP-7 and COMP-8
and the inventive radiation curable compositions INV-8 to INV-12 were
prepared according to Table 9. The same concentrated pigment dispersions
DISP-1 and DISP-2 of EXAMPLE 3 were used. The weight % (wt %) was based
on the total weight of the radiation curable compositions.

[0198] The comparative radiation curable compositions COMP-7 and COMP-8
and inventive radiation curable compositions INV-8 to INV-12 were coated
on PGA-paper, using a bar coater and a 10 μm wired bar. The coated
samples were mounted on a belt, transporting the samples under a Phoseon
4W 395 nm LED at a speed of 5 m/min and 10 m/min respectively. The degree
of conversion was determined and the results are summarized in Table 10.

[0199] From Table 10, it becomes clear that the initiators according to
preferred embodiments of the present invention result in a comparable
degree of conversion compared to thioxanthone based compositions with a
comparable composition (INV-8, INV-10, INV-12 and COMP-7; INV-9, INV-11
and COMP-8).

[0200] High contents of thioxanthone in radiation curable compositions are
known to give high sensitivity for LED curing but result in an instable
yellowing behaviour. Therefore, the comparative radiation curable
composition COMP-8, having a high ITX content, and the corresponding
inventive radiation curable compositions INV-9 and INV-11 were studied
more in depth for their yellowing behaviour.

[0201] The radiation curable compositions INV-9 and INV-11 and COMP-8 were
cured with a Phoseon 4W 395 nm LED, passing the sample 4 times under the
LED at a speed of 5 m/min. The stability of the image tone was quantified
by measuring the shift in b-value between the freshly printed sample and
the sample stored for 7 days at ambient temperature. The results are
summarized in Table 11.

[0202] From Table 11, it becomes apparent that the photoinitiators
according to preferred embodiments of the present invention have a
significant more stable yellowing behaviour compared to thioxanthones.

[0203] While preferred embodiments of the present invention have been
described above, it is to be understood that variations and modifications
will be apparent to those skilled in the art without departing from the
scope and spirit of the present invention. The scope of the present
invention, therefore, is to be determined solely by the following claims.